1. Field of the Invention
The present invention relates to a novel zeolite and to its preparation. In particular, the invention relates to a zeolite such as ZSM-5 which has cationic sites within the zeolitic pores and which has protonic sites external to the zeolitic pores. The novel zeolite is formed, for example, by contacting a Na-ZSM with a sterically hindered protonated salt which does not substantially enter the zeolitic pores whereby only cation on the surface external to the pores is exchanged for the proton.
2. Description of the Prior Art
There are a great number of patents which relate to crystalline zeolites, their production and use.
Reference can be made, for example, to the following U.S. Pat. Nos. which describe various zeolites: U.S. Pat. No. 3,702,886 which describes ZSM-5; U.S. Pat. No. 3,709,979 which describes ZSM-11; U.S. Pat. No. 3,832,449 which describes ZSM-12; U.S. Pat. No. 4,076,842 which describes ZSM-23; U.S. Pat. No. 4,016,245 which describes ZSM-35; and U.S. Pat. No. 4,046,859 which describes ZSM-38.
Generally speaking, the zeolites are prepared by crystallization and contain exchangeable cations, most notably sodium, distributed throughout. Since zeolite catalytic activity is due largely to acidic sites throughout the crystal, customarily the exchangeable cations throughout the structure are replaced by protons. The method most commonly employed for this exchange is contact with ammonium chloride which is effective in exchanging protons for cations located both on zeolite surfaces which are external to the pores as well as within the pores of the crystalline zeolite.
There are, however, processes where acid sites and consequent catalytic activity within the zeolite pores is not advantageous. An example of such a process is the conversion of higher saturated and olefinic hydrocarbons to low molecular weight olefins as set forth in copending application Ser. No. 07/343,097 filed April 25, 1989, now abandoned. Reactions occurring at acid sites within the pores promote the formation of carbon within these pores thus shortening the effective life of the zeolite.
For certain applications, a crystalline zeolite which has essentially only acid sites external to the pores offers distinct advantages. The present invention provides such a zeolite and its preparation.